1. Field of the Invention
The embodiments disclosed herein relate to a design method and structure for a transistor having a relatively large threshold voltage variation range due to exacerbated random dopant fluctuation (RDF). The embodiments also relate to a design method and structure for a random number generator, which incorporates multiple essentially identical transistors having such a large threshold voltage variation range.
2. Description of the Related Art
The multi-bit output of an on-chip random number generator may be used, for example, as a unique identifier for the chip, as a secret key, etc. Thus, it is important that the multi-bit output is stable (i.e., repeatable); however, achieving such stability can be difficult. Specifically, a random number generator typically comprises multiple pairs of essentially identical transistors. It can further incorporate multiple comparators, where each comparator receives voltage outputs from a corresponding pair of the transistors, performs a comparison of the voltage outputs and, based on the comparison, outputs a binary digit (bit) (e.g., a “1” or a “0”). The bits generated by all of the comparators can provide a random multi-bit output and, depending upon the number of pairs, this random multi-pit output may be very unique. Alternatively, rather than having one comparator for each pair of transistors, the random number generator can comprise a lesser number of comparators (e.g., only a single comparator) and one or more multiplexors that allow the voltage outputs from select pairs of transistors to be compared by the comparator for purposes of generating a unique multi-bit output (e.g., see U.S. patent application Ser. No. 12/823,278 filed on Jun. 25, 2010, assigned to International Business Machines, Inc. and incorporated herein by reference). In any case, such random number generators generally rely on detection, by a comparator, of threshold voltage mismatch in a pair of essentially identical transistors as a result of process variations in order to create each bit in the multi-bit output. However, if the threshold voltage mismatch is only minimal, detecting it can be difficult and the multi-bit output may not be stable (i.e., the value of some bits within the multi-bit output may change, for example, on environmental conditions at the time of detection).